chore: add proxy health interval flag/env config (#7919)

* chore: plumbing for proxy health interval from flag/env

cli/testdata/coder_server_--help.golden

@@ -193,6 +193,10 @@ backed by Tailscale and WireGuard.
           The maximum lifetime duration users can specify when creating an API
           token.
 
+      --proxy-health-interval duration, $CODER_PROXY_HEALTH_INTERVAL (default: 1m0s)
+          The interval in which coderd should be checking the status of
+          workspace proxies.
+
       --session-duration duration, $CODER_SESSION_DURATION (default: 24h0m0s)
           The token expiry duration for browser sessions. Sessions may last
           longer if they are actively making requests, but this functionality

cli/testdata/server-config.yaml.golden

@@ -33,6 +33,9 @@ networking:
     # directly in the database.
     # (default: <unset>, type: bool)
     disablePasswordAuth: false
+    # The interval in which coderd should be checking the status of workspace proxies.
+    # (default: 1m0s, type: duration)
+    proxyHealthInterval: 1m0s
   # Configure TLS / HTTPS for your Coder deployment. If you're running
   #  Coder behind a TLS-terminating reverse proxy or are accessing Coder over a
   #  secure link, you can safely ignore these settings.